The purpose of this work is to provide uniform, pre-defined and welldefinedmethod for the construction of RCC shell in chimney.Slip formwork system is a system in which the formwork is continuouslyraised byhydraulic equipment namely jacks by 1.5 to 3 Mtrs per day depending onthe rateof concrete setting. This act of slipping is a continuous process and thusthe name“SLIPFORM”.
The system uses steel panels of 1M height, which are fixed to workingcarcass consisting of steel frames (yokes), the yokes in turn are fixedto the trusses through spindles. A set of spindles connect the outerand the inner yokes and a set of spindles connect the yokes to theshutters. Carcass / structural framework provide stability and is a reliable basefor the wooden flooring (consisting of 2”X6”X6’ wooden planks madefrom hard wood) used as a working platform for placement ofconcrete, movement of reinforcement, etc. One hydraulic jack is placed on the lower head beam connecting theouter and the inner yokes. The jack climbs onto the jack rods that arefitted through the jack. The jack rods are embedded in the concrete ofthe shell through sleeves. The sleeves are fixed to the jacks. After acertain height the jack rods are recovered by Recovery jacks and thecylindrical openings left in the shell are grouted. The jack consists of a cylinder and a pair of upper and lower clampmechanisms gripping the rod and working in cycles.
The principle is that the jack works against the lower clamp to liftthe frame. When the pressure is released the upper clamp grips the rod andthe lower clamp is released and come up by spring action. Large capacity jacks are usually used with steel tubes of bigdiameters and greater stiffness. Concreting with slip formwork can be carried out on a continuousbasis, 24 hours a day, or with intervals. The slipform has three decks/ working platforms. They are :i. Upper Deck : Mainly to deliver concrete and fixingreinforcementii. Intermediate Deck : Used for vibration of concrete, adjustment ofspindles and operation and maintenance ofjacksiii. Lower Deck : Finishing and Curing
Once the shell is completed by the slipforming process, the structuralsteel beams and platforms are erected at various levels within thechimney. The flue cans are rolled & fabricated in segments of 2M – 2.5M widthwhich are brought separately to the inside of chimney on rail tracks.Three such segments (incase the width of each is 2M) or two suchsegments (incase the width of each is 2.5M) are assembled forinstallation. They are assembled at the vertical location of theirinstallation, welded and erected with hydraulic jacks which areinstalled on the roof of the chimney. After both the flue cans are erected, the mini shell is casted at the top.The mini shell can be either of RCC with acid resistant lining or ofstainless steel construction with insulation on the outside. External platforms, generally, at the rate of 40M intervals of RCCconstruction are constructed by workers supported by hangingplatforms through winches. All other miscellaneous works like painting, fixing of elevators andelectrical are completed.